Some cities have been looking at what are known as ‘hybrid‘ buses as ways to reduce air pollution, improve the attractiveness of their bus services and operate (partially) electric full-size buses without overhead wires.
Trials conducted with these buses between 2004 and 2006 have shown that as a general theme they are less polluting and use less fossil fuel than regular (diesel) mechanical buses, and based on this their advocates are trying to hoodwink the public into believing that use them would represent both the best / most advanced way forward which present day technology (in the public domain) can offer - as well as being 'the' solution to bus sourced air pollution.
It cannot be reiterated enough that hybrid buses are NOT 'ZEVs' (zero emission vehicles) and that as with ordinary 'mechanical transmission' buses they still pollute their local environments - ie: the streets
we use / the air we breathe.
The only buses capable of being true 'ZEVs' are battery electric, roadway power and overhead wire trolleybuses. These - along with electric trams, streetcars & light rail - do not give off any tailpipe pollution at all!
(These 'greener' technologies are explored on the 'Electric Buses' page.)
When thinking in the context of transport the term ‘hybrid‘ usually refers to vehicles which are powered from more than one power source. The concept is not new as some hybrid power transmission technologies actually predate WW1. In the present era it is usually used in the context of (road) vehicles that feature partial or full electric drive systems incorporating electrical energy storage (see below) and which typically use an onboard fossil fuel engine as their primary energy source. For the fossil fuel engines diesel seems to be fuel of choice although other possibilities include petrol, LPG, gas turbine, fuel cell or one of the commercially grown fuels eg: ethanol. As technologies change and local air pollution legislation become ever more stringent so the choice of primary energy fuels may change. For instance, gasoline (petrol) is also used in California in order to meet its very stringent air pollution regulations. This is because gasoline hybrids offered by one US manufacturer have been certified to lower emission standards for NOx and particulate matter than their hybrid diesel and even natural gas counterparts.
For energy storage batteries represent the 'tried, tested, understood' option. When hybrid buses first became popular the bus builders tended to opt for either lead-acid or nickel metal hydride (NiMH) batteries; at the present time both are used in some of the biggest hybrid bus fleets currently in service, however some bus manufacturers are also choosing Lithium-ion (Li-ion) batteries.
But batteries have reliability, durability, weight, cost and (when life-expired) environmental limitations so in an effort to find viable alternatives much money is being invested in other energy storage options. These include flywheels and super / ultra-capacitors (which are also sometimes known as ultracapacitors or in an abbreviated form, ultracaps).
In the late 1980's / early 1990's the German bus industry experimented with an electric bus which featured a small diesel engine coupled to a generator and a flywheel for energy storage. In 1992 a variant of this concept was realised in a small fleet of trolleybuses - ditching the still polluting fossil fuel engine for overhead wires. With these buses the flywheels absorb braking energy which is then released when accelerating, giving an impressive 23% saving in overall energy consumption. Flywheels can absorb regenerated energy much more quickly than batteries, are expected to last the entire life of the vehicle and can store enough energy to give the bus an estimated 1km of off-wire capability. However flywheels do not seem to have found favour, with some people suggesting they suffer from issues related to safety and gyroscopic forces. In the late 2000's another variant of this system saw a different bus builder introduce into its range some trolleybuses which followed the same concept except that the flywheel was replaced with ultra-capacitors to store the recycled energy.
As of 2009 ultra-capacitors seem to be becoming a favoured choice by some mainland European bus builders. Reasons are cited to include their expected life of twice that of even the most advanced batteries. Ultra-capacitors offer the promise of high power rates, lighter weight and long life at a reasonable cost. Their negative points include low power, bulkiness and a need for very complex electric management systems. Early experience has shown that they have trouble sustaining their charge during hill climbing, limiting their use for systems in hilly localities. As a contrast, overhead wire trolleybuses climb hills with relative ease.
Because they are much faster to absorb energy it should be possible for hybrid buses to use flywheels or super / ultra-capacitors to absorb regenerative braking energy and batteries to store energy from the fossil fuel engine. This would use the various energy storage technologies in the ways for which they are most suited.
Note that duo-buses are not hybrids, this is because whilst they feature both fossil fuel and electric traction packages they (normally) only use one at a time. Furthermore, they do not store their own electrical energy on the vehicle - instead they source it from twin overhead wires, as regular electric trolleybuses.
Thanks in part to British government funding an energy storage system which uses flywheels instead of batteries is also being developed. As of late 2009 the first prototype bus is being evaluated - not being a true hybrid bus (it does not combine electric with fossil fuel traction) this is not totally relevant for this page, however for the sake of completeness further information can be found below.
Generally ‘series‘ hybrids benefit from a greater flexibility in choice of locations for the fossil fuel engine afforded by the lack of a mechanical link between it and the wheels. However by having separate motor and generator portions (which can be combined in some parallel hybrid designs) the vehicle's effective energy efficiency will be lower than that of a conventional mechanical transmission motorbus, offsetting some of the efficiency gains that might otherwise be achieved. Series hybrids are most efficient in driving cycles that incorporate many stops and starts, such as is typical of urban bus routes. For longer distance highway driving the possibility exists of the motors drawing more power than the low power fossil fuel engine can provide and once the batteries have become exhausted the vehicle will only be able to operate at reduced power / low speed. Therefore for this type of operation the 'parallel' (or 'combined') types of hybrid may be more advisable. With series hybrids the power delivered to the wheels is limited by the electric motor(s) (which can be overloaded for a limited time however), whilst with parallel / combined hybrids both the fossil fuel engine and the electric motor(s) can provide power to the wheels simultaneously.
In Seattle (USA) the parallel hybrid buses include what is known as 'hush' mode. This is a special design feature for use when travelling through the city centre transit tunnel. When travelling in 'hush mode' these buses use electric traction below 10 mph (typically this is when in station areas) whilst above 10mph (which typically is when between stations) they use a special low power combination mode which sees the fossil fuel engine operating in reduced pollution / power mode.
It is possible for a hybrid bus to be designed to be 'charge depleting' which would mean that although there would still be a fossil fuel engine it would primarily be used to extend the vehicle's range, with the primary energy source coming from the mains power supply when the vehicle is 'plugged in', which could either be overnight or during the day. As with regular battery buses 'plug-in' hybrid buses would still need their batteries (or other energy storage devices) to be recharged on a regular basis.
In some ways this fossil fuel assist option could be said to be a merging of battery and 'series' hybrid bus technologies.
However for the most part transport operators seem to only want to use their existing diesel (etc.,) refuelling infrastructure, and therefore are not experimenting with the concept of 'plug-in' buses.
|The rear of a 2005 DE40LFR, owned and operated by the Washington Metropolitan Area Transit Authority (WMATA). Image sourced from the free online "Wikipedia" encyclopædia - link open in a new window.
|Hybrid bus in New York City. The slogan on the buses' side is totally misleading; to be accurate it should replace the word 'clean' with 'less dirty'.|
Although experience with hybrid buses is still very limited they do seem to have some advantages over conventional motor buses.
It should be remembered however that there would be even less pollution if 'series' type hybrid (and diesel-electric) buses were to be fitted with trolleypoles and only use the fossil fuel engines to extend services beyond the wires.
Apart from still polluting their local environment hybrid buses have a few other disadvantages too...
Will Battery Replacement Costs Create A Financial Disaster?
A potentially very major cost associated with hybrid buses is battery replacement.
Lead-acid batteries have an estimated life of approximately three years, during which time they will also benefit from periodic reconditioning. Being a mature technology replacement lead-acid batteries can be purchased 'off the shelf'. NiMH batteries are predicted to last longer than (between five and seven years) plus weigh less than lead acid batteries, which helps keep the bus within its legal weight limit without reducing the overall passenger capacity. However, NiMH batteries are more expensive to purchase 'upfront' and are expected to also be more expensive when replacement becomes due. Lithium-ion (Li-ion) batteries are much lighter than other types of battery and do not have a 'memory' effect - but a unique drawback is that they degrade (ie: decline slowly and predictably in "capacity") due solely to their age whether or not they are used.
The cost of replacement batteries may yet prove to be the 'Achilles heel' of hybrid buses. In the USA estimated costs of replacing the NiMH batteries are in the region of USD $260,000 per bus, which, if the cost is amortised over the full six year estimated life of the batteries works out a whopping $45,000 / year.
As yet (2009) hybrid buses are still too new to know whether the batteries will remain viable for as long as predicted. It may be that this will only be learnt the hard & expensive way.
The potential enormity of replacing batteries on buses fleetwide might yet make hybrids commercially unviable - that is, without government financial assistance. Dealing with the large number of waste batteries will also require some environmentally enlightened decisions. In the long term trolleybuses (or just removing the batteries and converting series hybrids to trolleybuses) might yet prove to be a cheaper option.
Information about hybrid buses has been sourced from the following websites and publications....
Despite their higher complexity, purchase costs and issues related to the extra weight - and ultimate disposal - of the batteries there are many advocates who see hybrid buses as offering some worthwhile advantages over traditional mechanical buses. Depending on the route served and the type of hybrid technology used ('series' / 'parallel' as detailed above) hybrid buses have the potential to be suitable for bus routes serving rural areas where air pollution is less of a severe issue, as well as infrequent urban services where fixed infrastructure simply cannot be justified (financially).
However, for busy urban services overhead wire electric trolleybuses remain the optimal solution, so that - as with trams / streetcars / light rail - there will be zero air pollution in the street. Experience in cities which operate trolleybuses and have investigated the costings have found that bus routes operating at frequencies of six vehicles per hour (ie: 10 minute headways) or greater are more economic to operate with trolleybuses. The advantages improve considerably when there are several routes which share some sections of overhead wiring.
The Electric Tbus Group has conducted a detailed study which suggests that for London the conversion of the busiest bus routes (such as those with a frequency of every 5 minutes or more) would offer significant financial and environmental benefits. Thanks to the network effect where multiple routes operate along the same roads the situation would soon arise whereby many subsequent conversions would entail less additional wiring - both increasing the cost effectiveness of existing wiring and reducing the cost of the electrification of additional routes.
Trolleybuses and other forms of electric buses are more fully explored on their own dedicated page.
Hybrid Buses In Britain.
In Britain between 2005 and 2010 the city of Newcastle-Upon-Tyne had a small fleet of hybrid buses which were sourced from New Zealand. In all there were 10 of these innovatively styled vehicles and they were dedicated to the high profile QuayLink services linking the waterfront with other parts of Newcastle and Gateshead. Costing £200,000 each, these were 'series' type hybrid buses which used an (approximately) 25kW gas turbine to charge solid gel, water-cooled batteries. In addition the batteries are also charged overnight. Each bus seated 30 passengers plus there was space for a wheelchair and 20-30 standing passengers.
The buses were withdrawn in 2010 with the cited reason being that they were too underpowered for the hillier sections of the bus routes. They were repaced with normal diesel buses.
The genesis of these buses was a partnership of Christchurch City Council, local bus company Redbus and New Zealand bus builder Designline, with the aim of creating an electric bus that would help reduce air and noise pollution in Christchurch. With
batteries not being a viable option for a full day's service so hybrid technology was chosen as an alternative option which would at least partially meet their aspirations. In 1998 these buses started operating a high frequency free inner-city centre shuttle
service that became so popular with passengers that it was even credited as creating a positive image for bus transport which spread fleetwide. This service ended as a result of the February 2011 Christchurch earthquake which caused much destruction in the
city centre and - because the factory was also in the area affected ny the earthquake - also caused the demise of the company which built them (Designline).
|Hybrid electric QuayLink buses in Newcastle-Upon-Tyne.|
|above left: Passengers pay the driver of a vehicle on route Q1 whilst at the bus stop outside the Central Railway Station.
above right: A QuayLink bus on route Q2 in Newcastle city centre. Note the lack of rear window - as that is where much of the equipment is located.
left: The promotional slogan as seen on the buses. Note how the city names (NewcastleGateshead) are effectively one word.
Also in Britain, in summer 2006 six UK-sourced 'series' hybrid buses were introduced on a central London route which is shared with ordinary motor buses. At the time media reports suggested that each of these experimental buses cost £40.000 more than normal motor buses. The buses are midi sized, with a total passenger capacity of 57, of which 26 are seated. They are fitted with car sized 1.9 litre turbo diesel engines which are coupled directly to a generator. Energy storage comes in the form of 2 banks of 14 lead acid batteries which are mounted each side at the rear of the bus, and add 900kg to the vehicles' overall weight. Provision has been made at the bus garage for overnight booster charging of the batteries, although this will only be done once felt desirable. Testing on an off-road circuit prior to use in London found an 40% improvement in fuel consumption. early trials in London found that they suffered from overheating during the hottest part of the summer and therefore for a while they had to be withdrawn from service whilst this was rectified.
|Bus No. WHY5 which is number five of the six diesel - electric hybrid buses being trialed on route No. 360 in London, summer 2006.
The view above left shows the vehicle that had been travelled on trying to pull away from a bus stop outside South Kensington underground station. The vehicle's destination (Elephant & Castle) is a part of London where people with cameras need to be very very careful.
The other views were taken at the Kensington Gate terminus which is also used by the Heritage Routemasters on route No. 9h.
As the logo above the front of the bus in the view above right suggests, these trials are being carried out with the approval of London's Mayor. The vehicle designation code ("WHY") speaks volumes.
Notice the lack of rear view window, which is necessary in order to fit in all the mechanical components without intruding too much into the passenger saloon. Because it also means that passengers will not be able to see if the bus they want is close behind (or otherwise) the lack of rear window can make changing to other buses somewhat user-unfriendly.
The slogans on the back & side of the bus which talk about it being powered by "cleaner" electric hybrid technology would be equally truthful if the word "cleaner" was to be replaced with "less dirty".
The travelling experience on these hybrid buses was different to regular motor buses in that the usual engine noise (which varies according to speed) was replaced by a flat droning background engine noise, which the bus driver described as being akin to that of the aircraft engine noise heard by passengers whilst flying. Depending on where a passenger is standing or sitting it was sometimes possible to feel vibration from the engine too. Noise apart the electric drive system made this bus smoother to ride in than regular motor buses, plus there was considerably less "rattle" when stationery. If sitting at the very back of the bus there was a slight diesel smell and the warmth of the engine could also be felt. NB: since this sample ride the buses have been rebuilt, which explains the 'past tense' nature of this paragraph.
Click the speaker symbol or here to download a 426kb mono soundclip (named inside-hybrid-bus.mp3) which was recorded using a mobile telephone and demonstrates the engine noise as heard whilst sitting at the back of the bus.
In 2007 more single deck hybrid buses were introduced on a second bus service, in the south-east of London.
Double-deck Hybrid Buses.
At the end of October 2006 the same British-based bus manufacturer which built the single-deck hybrid buses seen above developed a prototype double deck hybrid bus. At around the same time London's Mayor publically stated his interest in the vehicle, and that once full-scale production begins he would like to see as many as 500 a year being introduced in London, replacing existing fossil fuel motor buses.
This prototype double deck hybrid bus entered passenger service in March 2007, and even before it had been extensively tested it was announced that another 10 similar vehicles were to be built, with one going to Dublin, the capital of the Irish Republic, and the rest going to London.
Although the double deck hybrids use the same 1.9 litre turbo diesel engines as the single deckers, they feature different battery technology. Instead they use 30 20v lithium ion batteries which provide a nominal 600v output, have a longer life and both give / take charge much more quickly. In addition their 455kg weight is far less than lead acid batteries, which had they been used would have added another 1.5tonne to their overall weight. Most of the time the batteries provide tractive force, however when accelerating hard the diesel engine provides power too. Although the diesel engine runs at a constant speed, this speed is increased if the batteries are becoming depleted. Following experience with the single deck hybrids the double deckers cannot be driven on battery power alone if the diesel engine runs out of fuel.
Perhaps unwittingly the creation of a series hybrid double bus means that the introduction of trolleybuses is one step easier; this is because trolleybuses and series hybrid buses both use an 'all electric' drive train. It is how the electricity is sourced (diesel engine or overhead wires) which differentiates the two.
|Just a few weeks after its introduction in to passenger service the first hybrid double-decker is seen at the bus station outside London Bridge railway station.
At this time the bus was making just three return trips daily.
These images were sourced on Friday 13th April 2007.
As an aside, these are not London's first 'ever' hybrid buses. In 1986/7 there were trials of an experimental hybrid bus which captured braking energy that was then recycled to accelerate the bus to speeds of 20mph - with the diesel engine remaining at idle (ie: as if the bus was at a bus stop / red traffic signals).
In December 2008 a 'road map' for expanding the use of hybrid buses over a period of several years was announced, however since the purpose of this page is to look at the vehicles and their technology rather than exactly how many, where, when, (etc.,) so this page will not be updated with these details.
In late 2009 the government announced which bus companies / operators had won funding through its 'Green Bus Fund' for hybrid and pure electric buses throughout Britain. Apparently the competition's prize money was £30.2 million and this will fund as many as 349 buses, only 46 of which are destined for London. Woweee! Ah well, the areas which receive the few crumbs that fell off the table will at least gain real-world experience with these types of buses.
As time passes some of the information above has become out of date. For instance the single deck buses used on London's route 360 have been substantially rebuilt with different 4.5 litre diesel engines and lithium-ion batteries - pushing their weight up by about 500kg. In addition, as more bus manufacturers embrace hybrid technology so more hybrid buses continue to be introduced in London, plus to a certain extent, in provincial conurbations too. Some of these newer buses use parallel drive systems with the diesel engine switching off when the bus stops and coming back on once the bus has reached about 12.5mph (20km/h) or when the diminishing stored energy dictates. These buses can also be driven in 'diesel only' mode, should the energy storage system fail.
Since the intended purpose of this website is to provide an overview of a wide range of public transport technologies rather than look at every latest development in fine detail so this page will not be recording every change in bus fleets.
In late summer 2007 a British company which already has some hybrid buses being evaluated by bus operators in various parts of Britain announced pioneering plans for the testing of an innovative triple system bus using lead acid batteries, a 1.9 litre Volkswagen diesel engine and supercapacitors. The trials by (bus operator) Arriva Southern Counties in Horsham, Sussex, are scheduled to commence in October.
It is being suggested that for uses where there is no requirement to provide zero emissions, the use of supercapacitors could render the batteries redundant. Hopefully the people of Horsham will not object to learning that their area is not deemed to merit clean air free from diesel engine derived pollution.
As an aside, according to the free online "Wikipedia" encyclopædia a definition of the name 'Tribrid' is the third generation of alternative propulsion vehicles. Second generation are the bivalent hybrid vehicles with turbines. Tribrids are hybrids that obtain additional energy from the ambient environment (solar panel, windmill or sail). An example is a velomobile with an electric power-assist motor and additional on-board solar cells.
There is also a specialist website about Tribrids, which defines them thus...
Tribrids work in much the same way as hybrids, except that some form of the stored or generated energy either has been, or is being taken, from the ambient environment. A free-energy capturing device such as a solar panel, windmill or sail provides the tribrid vehicle with a source of power that, if left un-captured, is otherwise simply lost to the atmosphere.
http://www.tribrids.com/ (links lead to external sites and open in new windows).
Also worthy of note is the Flybus project which is being sponsored by the government with the aim of creating a bus with a mechanical energy storage drive system.
Instead of batteries the bus features a modified Allison type hybrid system which uses continuously variable transmission (CVT) and a high-speed composite flywheel to store braking energy and release it upon acceleration.
The aim is for the bus to use a minimum of 20% less fuel than a standard diesel bus.
The system's developers claim that it is half the size, half the weight and quarter of the cost of diesel-electric hybrid systems. Some of these savings will be because it does not need batteries.
In addition to being factory fitted on new buses it is also claimed that this system could be retrofitted to existing buses, at a modest cost.
However, it does mean that the bus remains diesel mechanical at all times, so it is questionable whether it can be known as a 'hybrid'?
The first demonstrator midibus was built in late 2010 and the first results of the trials are expected in 2011.
Direct links to other Buses pages...